Physical Properties of Inorganic Solid Surfaces

Many of the surface properties of a solid depend on the electrical and thermodynamic potentials that develop at its surface, and control of those potentials is basic for the design of functional materials. The consequences of these potentials are quite different for an ionic solid, a covalent semiconductor, or a metal. 

Particles of ionic insulators suspended in solution can have charged surfaces because of an excess there of ions of one type as a result, e.g., of adsorption from the solution. The surface charge depends on the concentration in the solution of ions that can adsorb. The sol-gel technique is discussed in the next section as an example of a synthesis of nonmetallic inorganic polycrystalline materials that makes use of surface charge control of colloidal particles. 

The energy of valence electrons in a metal and on the surface depends on nonlocal factors such as the existence of certain facets elsewhere on the crystal. The bonding energy of valence electrons of adsorbed molecules also depends on the potential at the surface where the molecules are adsorbed and that potential depends on planes elsewhere on the crystal. The absolute bonding energy of electrons of adsorbed molecules can only be measured with respect to the Fermi level of the solid. 

The large effect of bulk doping and Fermi level on surface reactivity is seen when n-type and p-type silicon single crystals are etched. A high Fermi level in the silicon (upward curling band edges) etches with formation of clean crystal planes, while the surface of anodically etched p-type silicon is ramified and highly porous. 

The chemical activity of a metal is strongly influenced by the potentials at its surface. This has also been observed in heterogeneous catalysts that consist of metal particles on an ion-conducting ceramic substrate. Such a substrate makes it possible to change the potential of the metal particles. The reactions studied were the 

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